Conventionally, the hull of a liquid carrying ship such as an LNG (Liquefied Natural Gas) carrier is equipped with at least one liquid storage tank. In the case of an LNG carrier, as shown in FIG. 13, a spherical tank 3 formed of an aluminum alloy for use in storing LNG in an extremely low temperature (−163° C.) is fixed to a hull 2 which is formed of for example, a steel or stainless steel. The spherical tank 3 is fixed to the hull 2 via a cylindrical support structure called a tank skirt 4.
The upper part of the tank skirt 4 is joined to the tank 3 by welding, and the lower part of the tank skirt 4 is joined to the hull 2 by welding. Therefore, the tank skirt 4 includes: an upper structural member 41 formed of the same material as that of the tank 3 (here, an aluminum alloy); a lower structural member 42 formed of the same material as that of the hull 2 (here, a steel); and an intermediate structural member 43 disposed between the upper structural member 41 and the lower structural member 42. The intermediate structural member 43 is formed of a material having low thermal conductivity, for example, a stainless steel. A steel and a stainless steel can be joined together by welding. However, since the melting point of a stainless steel is significantly different from the melting point of an aluminum alloy, it is difficult to directly join these dissimilar materials together by welding. In addition, it is known that if these materials are mixed together in a melted state, the material strength is reduced significantly. Therefore, the upper structural member 41 and the intermediate structural member 43 are joined together via a dissimilar material joint 50. The upper end of the dissimilar material joint 50 is joined to the upper structural member 41 by welding, and the lower end of the dissimilar material joint 50 is joined to the intermediate structural member 43 by welding. Patent Literature 1 discloses one example of the dissimilar material joint 50.
According to Patent Literature 1, as shown in FIG. 14, the dissimilar material joint 50 includes: a first member 51 formed of the same material as that of the intermediate structural member 43 (here, a stainless steel); a nickel member 52; a titanium member 53; and a second member 54 formed of the same material as that of the upper structural member 41 (here, an aluminum alloy). The first member 51, the nickel member 52, the titanium member 53, and the second member 54 are stacked and fixed to each other, and thus integrated together. The dissimilar material joint 50 is fabricated in the following method: first, the nickel member 52 is placed on the first member 51, and the first member 51 and the nickel member 52 are joined together by explosive welding; next, the titanium member 53 is placed on the nickel member 52, and in such a state, the nickel member 52 and the titanium member 53 are joined together by explosive welding; and at last, the second member 54 is placed on the titanium member 53, and in such a state, the titanium member 53 and the second member 54 are joined together by explosive welding. It should be noted that explosive welding is alternatively referred to as “explosive bonding”, which is a method of joining two kinds of metals together by causing the metals to collide with each other at high speed with explosive force.
The dissimilar material joint 50 fabricated by the above-described method is placed on the intermediate structural member 43 of the tank skirt 4 in a process of producing the tank skirt 4, and the lower end of the dissimilar material joint 50 and the upper end of the intermediate structural member 43 are fillet-welded. Further, the upper structural member 41 is placed on the dissimilar material joint 50, and the upper end of the dissimilar material joint 50 and the lower end of the upper structural member 41 are fillet-welded. In this manner, the upper structural member 41 and the intermediate structural member 43, which form the tank skirt 4, are integrated together via the dissimilar material joint 50.